1. Field of the Invention
The present invention relates to a stablized power-supply circuit, and more specifically to a stabilized power-supply circuit which is supplied with electric power from an external power source and to supply a stabilized voltage to a load having an auxiliary power source.
2. Description of Related Art
Stabilized power-supply circuits have been widely used for supplying a stable electric voltage to electronic devices such as microcomputers, semiconductor memories, etc. When the output voltage supplied from the stabilized power-supply circuits accidentally drops, the microcomputers and semiconductor memories are often inadvertently cancelled or lost, and information stored in memories is garbled in some cases. For avoiding such troubles, an auxiliary electric source is ordinarily connected in parallel with a load such as a microcomputer and a memory, so that the auxiliary electric source will supply the necessary electric power to the load when the voltage of a main power source drops. But, the auxiliary electric source generally has only a relatively small capacity, and therefore, in order to make it possible for the auxiliary electric source to supply a necessary voltage to the load for as long a time as possible, it is necessary to prevent an electric current from flowing to the stabilized power-supply circuit from the auxiliary electric source. If a blocking diode is provided for this purpose, the voltage actually supplied from the stabilized power-supply circuit to the load is lowered by a forward voltage drop of the diode in a normal operation. Therefore, the of such a blocking diode is not suitable because of a restriction of effective utilization of the main power source in as wide a voltage range as possible.